Children in the same family often differ significantly in psychological characteristics and behavioral outcomes. They also show significant resemblances. Although these facts are well known, the processes by which family environments are linked to sibling differences and similarities are only partly understood. Such understanding is critical to developmental theory and to theories of gene-environment interactions. The purpose of this study is to investigate the shared and nonshared family experiences of siblings in relation to their similar and different social and emotional development. The research has three objectives. The first is descriptive what are the family experiences of siblings? The second concerns the links between siblings' shared and nonshared family experiences and the developmental course of each sibling. The third is to investigate child-specific attributes (temperament, gender, age, and others) that interact with environment and with environmental influences on sibling development. This study is at a beginning stage.